As gas turbine engine technology advances and engines are required to be more efficient, gas temperatures within the engine continue to rise. However, the ability to operate at these increasing temperatures is limited by the ability of the superalloy turbine blades and vanes to maintain their mechanical strength when exposed to the heat, oxidation, and corrosive effects of the impinging gas. One approach to this problem has been to apply a protective thermal barrier coating which insulates the blades and vanes and inhibits oxidation and hot gas corrosion.
Typically, the thermal barrier coating will have an outer ceramic layer that has a columnar grained microstructure. Gaps between the individual columns allow the columnar grains to expand and contract without developing stresses that could cause spalling. Strangman, U.S. Pat. Nos. 4,321,311, 4,401,697, and 4,405,659 disclose a thermal barrier coating for a superalloy substrate that contains a MCrAlY layer, an alumina layer, and an outer columnar grained ceramic layer. Duderstadt et al., U.S. Pat. No. 5,238,752 and Strangman copending U.S. patent application Ser. No. 06/603,811, now U.S. Pat. No. 5,514,482, disclose a thermal barrier coating for a superalloy substrate that contains an aluminide layer, an alumina layer, and an outer columnar grained ceramic layer.
A problem with columnar grained ceramic layers is that when exposed to temperatures over 1100.degree. C. (2012.degree. F.) for substantial periods of time, sintering of the columnar grains occurs. The gaps close as adjacent columnar grains bond together. Once the gaps become closed, the ceramic layer can no longer accommodate the thermal expansion and may spall or crack.
Accordingly there is a need for a thermal barrier coating having a columnar grained ceramic layer that is resistant to the sintering of the grains.